feat: brain/index.go shebang; mail import is not a brain write (D14).

Commands live at bin/brain/{index,get,stats,eval,watch}.go and
bin/mail/import.go, bin/markdown/import.go, bin/postgres/query.go.
Python remains the Ladybug write worker. index_mail is a deprecation
shim that rebuilds via --with-mail.

bin/brain/watch.go

@@ -0,0 +1,20 @@
+//usr/bin/env go run -tags=brain_watch "$0" "$@"; exit
+//go:build brain_watch
+//
+// bin/brain/watch.go - re-index when corpus files change.
+//
+//	./bin/brain/watch.go [dir...]
+//	KB_WATCH_INTERVAL=15 ./bin/brain/watch.go
+//
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/bin/watch"
+)
+
+func main() {
+	watch.Run(os.Args[1:])
+}